1. Field of the Invention
The present invention relates to a device with electrical, mechanical and aerodynamic features that enable it to convert the energy in the wind to usable electrical power.
2. Description of the Prior Art
As the supply of conventional fossil fuels decreases and their costs increase, modern industrial society will turn increasingly to alternate sources of energy. Of these, windpower is one of the most promising since it is abundant, widely distributed and environmentally benign. It has also established its ability to meet the needs of society through usage over many hundreds of years and in many parts of the world. In today's industrial society, however, windpower will be accepted as a satisfactory replacement for conventional fuels only if it is convenient to use and low in cost. Both of these conditions are satisfied if the wind machine is simple in construction, aerodynamically efficient and capable of unattended operation.
Since the desired blade angle or pitch of horizontal axis wind turbines varies with wind speed and rotational speed, it has been common practice to support the turbine blades in bearings that allow the blades to rotate about their longitudinal axis. In this way, the blade angles may be changed through suitable mechanical linkages and control systems that sense the need for a change in blade angle and provide the necessary input signals. Thus, in the past, windmills for electrical generation have gained efficiency at the cost of aerodynamic, mechanical and control system complexity.
Conversely, simple windpower machines with rigid fixed-pitch blades operate at their highest efficiency only at a single combination of wind speed and rotational speed. At all other combinations, the blade angle is either too high or too low. As a result, fixed-pitch turbines operate at relatively low efficiency, require high start-up windspeeds, and have little inherent tendency toward self-governing at high wind speeds.
Thus, the alternatives have been, on the one hand, relatively efficient machines that are complex, expensive, and difficult to maintain, and, on the other hand, relatively inefficient machines that are simple, inexpensive and difficult to control at high wind speeds.
Furthermore, the disparity between the relatively slow rotational speed for wind turbines and the high rotational speed for most electrical generators has led to the use of gears, belts or other speed-increasing devices between the turbine and the generator. The resulting complexity has tended to increase the initial costs and the maintenance requirements for windpower generators of this type.
The present invention overcomes these problems to a large extent by providing a variable pitch turbine blade assembly having no moving parts, and by eliminating all gearing between the turbine blade assembly and the electrical generating system.
The turbine blade assembly will start-up at low wind speeds, and is self-governing at high wind speeds. This is accomplished by the use of a plurality of torsionally flexible cambered sheet airfoil blades rigidly mounted at their radially inner ends to a central hub. These blades have an out-of-plane balance weight attached near the radially outer end of their leading edge. A component of the centrifugal force acting on this out-of-plane balance weight acts to twist the blades about their respective radial axes so as to decrease the pitch of the blades with increasing rotational speeds. The system is constructed so that a radially outer portion of the blades will stall at a predetermined rotational velocity thereby causing the system to be self-governing at that velocity.
Out-of-plane balance weights have previously been used with torsionally non-flexible blades having a short torsionally flexible connecting means located between a radially inner end of the blade and the hub. In those apparatus the out-of-plane weight has typically been connected adjacent the connection of the radially inner end of the blade and the torsionally flexible connecting means. In such apparatus, the change in blade pitch which occurs with varying rotational speed is a uniform change along the entire radial length of the blade, as opposed to the continuously varying change in blade pitch which occurs with the present invention. An example of such a prior art device is disclosed in Cheney and Spierings, "Self-regulating Composite Bearingless Wind Turbine", report No. C00/2614-7612 (September 1976) prepared for the United States Energy Research and Development Administration under Contract No. E(11-1)-2614. Another apparatus similar to that of Cheney and Spierings is disclosed in Wind Power Digest, No. 12, pp. 7-9 (Spring/Summer 1978), although sufficient facts are not presently known to us to determine whether this latter disclosure, or the apparatus referred to therein, constitutes prior art with respect to our invention.
The elimination of gearing between the turbine blade assembly and the electrical generating system is achieved in the windmill of the present invention by the use of an alternator comprising a stationary multiple pole wound stator, and a multiple permanent magnet structure rotatably mounted coaxial with the stator. The permanent magnet structure is rigidly attached to the rotating turbine blade system so as to rotate therewith, thereby eliminating the need for drive gears or the like between the turbine blade system and the electrical generating system. Similar alternators, used singly, that is without coaxially stacking a plurality of alternators, are known to the art as described in T. Carmichael, "Ignition and Battery Charging with Permanent Magnet Alternators", SAE paper no. 670046 presented at Automotive Engineering Congress, Jan. 9-13, 1967, in Detroit, Mich.